U.S. Pat. No. 3,269,561, to J. DeLigt, discloses a latching mechanism of the general type to which the present invention relates and exemplifies some of the problems that are solved by the invention and the disadvantages that are avoided or overcome by it. As brought out in the DeLigt patent, a typical application for such a mechanism is in a stacker crane used for placing articles onto and removing them from vertically spaced storage shelves or the like. A stacker crane comprises a bridge-like trolley which runs along laterally spaced overhead tracks and from which a rigid, relatively fixed inner mast member projects downward. An intermediate mast member is slidable up and down in telescoping relation to the inner mast member, for effectively increasing its downward extension. A load carrying member, which can comprise a horizontally projecting fork, is guided for up and down motion on the intermediate mast member.
The latching mechanism operates in such a manner that the load carrying member, when in the lower part of its range of up and down motion, is effectively locked to the bottom portion of the intermediate mast member, which therefore moves up and down with the load carrying member and is guided on the inner mast member. In an upper portion of the range of motion of the load carrying member, the intermediate mast member is effectively locked to the inner mast member, and the load carrying member moves up and down realtive to both of those mast members. The latching mechanism thus enables the load carrying member to move through most of the distance between the trolley and the floor of the area traversed by the crane, while the mast never extends any farther down than is necessary to dispose the load carrying member at the level where it is needed, thus minimizing the chances for a collision between the crane and articles on the floor beneath it.
With the type of mechanism here under consideration, the intermediate mast member of a stacker crane is always supported either by the load carrying member or by the inner mast member, and therefore only the load carrying member has to have a direct connection with the hoisting drum or windlass. The latching mechanism changes the locking connection automatically as the load carrying member moves through a small zone intermediate the limits of its range of motion. As the load carrying member traverses that zone, the intermediate mast member can be transiently locked to both the load carrying member and the inner mast member, but there obviously cannot be an instant when the intermediate member is disconnected from both of those other members and is thus free to drop.
The automatic latching mechanism disclosed in the DeLigt patent comprised a toggle element that was carried by the intermediate mast member and was swingable relative to it between a pair of defined locking positions. In one locking position the toggle element engaged an abutment on the inner mast member; in the other it engaged an abutment on the load carrying member. Under the weight of the intermediate member, the toggle element tended to swing away from each of its locking positions, and therefore automatically operating means had to be provided for confining the toggle element in each locking position, for releasing it from such confinement when it was to flip over to its other locking position, and for again releasably confining it when it attained its other locking position. The means for effecting such confinement and release at the proper times comprised a slidable latching element that was carried by the intermediate mast member for movement relative to it between toggle latching and toggle releasing positions. The latching element was actuated by cams that were respectively fixed on the inner mast member and on the load carrying member, and those cams had to have substantial vertical extension to ensure that the latching element would always maintain its proper position through all movements of the telescoped members relative to one another. When the latching element was in its latching position, the toggle element engaged it under bias, and such bias had to be relieved before the latching element could be shifted to its releasing position. Hence there had to be a second set of cams, respectively carried by the inner mast member and the load carrying member, which cooperated with the toggle element for relieving its force against the latching element.
As is apparent from the foregoing brief description, the mechanism of the DeLigt patent was complicated, in that it comprised numerous parts, some of which were rather cumbersome and all of which had to be assembled in rather accurate relationships to one another. As a result, the mechanism tended to be more expensive than the conventional counterweight for the intermediate mast member--which it was intended to supplant--and it seems to have had no significant commercialization.
The DeLigt patent brings out that simplicity, compactness and low cost were among the objects sought to be achieved by means of the structure disclosed in it, and that structure was evidently the utmost in simplicity, compactness and low cost that the patentee was able to attain, notwithstanding the ingenuity and the high degree of skill in the art that the patent otherwise demonstrates.